Metal halide arc discharge lamps are widely used for general illumination. These lamps include an arc tube mounted within a light-transmissive lamp envelope. The lamp envelope may be evacuated or backfilled with nitrogen. The arc tube has electrodes sealed therein at opposite ends and contains a fill material including a starting gas, mercury and one or more metal halides. A starting electrode is commonly positioned at one end of the arc tube adjacent to one of the main electrodes to assist in starting. When the lamp is first turned on, a voltage is applied between the starting electrode and the adjacent main electrode. A discharge between the starting electrode and the adjacent main electrode heats the arc tube sufficiently to form a discharge between the main electrodes. After a discharge is formed between the main electrodes, the starting electrode is shorted to the adjacent main electrode by a thermal switch.
A thermal switch for metal halide lamps is disclosed in U.S. Pat. No. 3,965,387, issued Jun. 22, 1976 to Stuart et al. The disclosed thermal switch includes a bimetal element and a spring element attached to the bimetal element. At temperatures above the closure temperature, the spring is deformed, thereby relieving some of the stress which would otherwise be applied to the bimetal element.
A thermal switch for an arc discharge lamp is disclosed in U.S. Pat. No. 4,659,965, issued Apr. 21, 1987 to Bonazoli. The disclosed switch includes a bimetal strip attached to one electrode lead and a spring-like member attached to another electrode lead. The bimetal strip has a notch to receive the spring-like member when the closure temperature is reached.
While the prior art thermal switches provide generally satisfactory operation, they are subject to certain to practical problems. When temperature in the discharge lamp exceeds the closure temperature, the elements of the thermal switch are stressed and tend to become permanently deformed after multiple operations. When the switch elements are permanently deformed, the closure temperature of the switch changes and, in some cases, the thermal switch may fail to operate. Furthermore, it is frequently difficult to mount the elements of the thermal switch in the discharge lamp with sufficient precision to insure the desired operation. When the switch elements are displaced even slightly from their design positions, switch closure occurs at a temperature different from the desired closure temperature.
It is a general object of the present invention to provide improved thermal switch assemblies for use in electric lamps.
It is another object of the present invention to provide improved arc discharge lamps.
It is a further object of the present invention to provide improved thermal switch assemblies for controlling the application of electrical energy to a starting electrode in a metal halide arc discharge lamp.
It is yet another object of the present invention to provide a thermal switch assembly having a mounting frame to facilitate installation of the switch assembly.
It is a further object of the present invention to provide a thermal switch assembly which has a long operating life.
It is another object of the present invention to provide a thermal switch assembly wherein the switch elements are not significantly stressed during operation.